West Grand High School
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the West Grand High School clear sky chart from 2005-11-06 to 2025-07-06.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	2 6% 3 7% 5% 5% 6% 5% 7% 9% 45%	124080
	Night	2 6% 3 8% 5% 5% 5% 5% 8% 8% 45%	123650
Transparency	Day	31% 9% 13% 23% 18% 6%	123832
	Night	31% 9% 14% 25% 16% 5%	123486
Seeing	Day	9% 16% 27% 28% 17% 4	16478
	Night	6% 8% 27% 41% 17% 1	26551

By Month

Percentage Cloud Cover SW (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	7% 6% 15% 7% 7% 7% 5% 8% 8% 28%	3638
	11th to 20th	Day	8% 4 14% 8% 7% 7% 5% 7% 8% 31%	3936
	21st to end	Day	7% 4 10% 6% 7% 5% 4 7% 9% 40%	4510
February	1st to 10th	Day	1 12% 4 11% 8% 5% 5% 5% 6% 8% 35%	1800
	11th to 20th	Day	8% 5% 10% 6% 7% 7% 6% 8% 7% 36%	1840
	21st to end	Day	1 8% 4 12% 7% 6% 6% 5% 8% 7% 37%	1650
March	1st to 10th	Day	1 6% 4 8% 6% 5% 6% 5% 8% 7% 45%	4656
	11th to 20th	Day	7% 3 8% 5% 5% 5% 5% 7% 10% 43%	5000
	21st to end	Day	1 8% 3 11% 7% 7% 7% 6% 8% 9% 34%	5678
April	1st to 10th	Day	2 10% 3 7% 5% 4 5% 4 7% 9% 44%	2400
	11th to 20th	Day	3 9% 3 10% 7% 5% 6% 4 8% 7% 37%	2371
	21st to end	Day	3 10% 3 8% 5% 6% 6% 5% 8% 8% 39%	2400
May	1st to 10th	Day	2 9% 4 8% 7% 6% 7% 6% 8% 9% 33%	5804
	11th to 20th	Day	3 6% 2 7% 5% 7% 7% 6% 7% 9% 41%	6298
	21st to end	Day	3 5% 2 5% 5% 6% 7% 6% 8% 9% 45%	6925
June	1st to 10th	Day	3 4 2 5% 4 5% 5% 4 7% 10% 52%	2800
	11th to 20th	Day	3 3 1 4 3 4 5% 4 6% 9% 58%	2800
	21st to end	Day	2 3 1 4 4 4 5% 4 7% 8% 58%	2800
July	1st to 10th	Day	3 3 1 4 3 4 5% 5% 6% 9% 58%	4501
	11th to 20th	Day	2 2 1 5% 5% 6% 8% 6% 9% 12% 45%	4430
	21st to end	Day	3 3 1 5% 5% 6% 6% 5% 7% 8% 51%	4557
August	1st to 10th	Day	4 4 2 5% 5% 6% 7% 6% 9% 11% 41%	4092
	11th to 20th	Day	3 4 2 5% 5% 5% 7% 6% 8% 10% 45%	3853
	21st to end	Day	3 3 1 4 5% 5% 6% 4 7% 10% 52%	4180
September	1st to 10th	Day	4 3 1 4 4 4 4 4 6% 8% 57%	2280
	11th to 20th	Day	3 3 2 5% 3 3 5% 4 6% 8% 56%	2280
	21st to end	Day	5% 4 1 4 3 3 3 2 5% 6% 63%	2041
October	1st to 10th	Day	3 4 1 3 3 3 3 2 3 4 70%	3170
	11th to 20th	Day	3 5% 1 4 3 4 5% 5% 7% 9% 55%	3150
	21st to end	Day	4 7% 2 7% 4 4 4 4 5% 7% 52%	3438
November	1st to 10th	Day	2 7% 2 5% 4 3 4 4 6% 7% 55%	1763
	11th to 20th	Day	2 9% 3 6% 5% 4 3 4 5% 7% 52%	1789
	21st to end	Day	2 7% 3 9% 4 4 4 3 5% 6% 54%	1600
December	1st to 10th	Day	1 5% 2 9% 5% 4 4 5% 10% 6% 48%	2620
	11th to 20th	Day	6% 3 7% 5% 4 5% 5% 7% 9% 47%	3026
	21st to end	Day	1 7% 4 12% 7% 7% 7% 6% 8% 8% 32%	4004
January	1st to 10th	Night	8% 6% 13% 8% 6% 7% 7% 9% 8% 27%	6327
	11th to 20th	Night	1 8% 4 14% 12% 7% 6% 5% 8% 7% 29%	6087
	21st to end	Night	1 10% 5% 12% 7% 6% 6% 6% 11% 6% 30%	6468
February	1st to 10th	Night	1 10% 6% 12% 6% 4 6% 5% 6% 7% 35%	2600
	11th to 20th	Night	1 9% 5% 11% 6% 4 5% 5% 8% 9% 37%	2447
	21st to end	Night	1 8% 6% 12% 7% 6% 4 4 6% 6% 40%	1980
March	1st to 10th	Night	1 11% 4 10% 5% 4 4 5% 9% 7% 41%	5208
	11th to 20th	Night	1 7% 3 7% 5% 4 5% 6% 8% 7% 46%	5000
	21st to end	Night	2 8% 4 9% 5% 5% 6% 5% 8% 9% 40%	5274
April	1st to 10th	Night	3 8% 3 7% 5% 4 4 4 6% 8% 48%	2000
	11th to 20th	Night	4 9% 3 8% 5% 5% 4 4 6% 8% 43%	1977
	21st to end	Night	5% 10% 3 5% 3 4 4 3 6% 8% 51%	2000
May	1st to 10th	Night	3 7% 3 7% 5% 5% 6% 5% 7% 10% 42%	4171
	11th to 20th	Night	5% 6% 2 5% 4 5% 6% 4 7% 10% 45%	3617
	21st to end	Night	3 4 2 4 4 5% 6% 5% 6% 9% 52%	3972
June	1st to 10th	Night	3 2 1 3 3 4 4 3 7% 9% 62%	1600
	11th to 20th	Night	3 2 1 4 3 3 4 4 5% 7% 64%	1600
	21st to end	Night	2 2 1 3 3 3 5% 4 6% 9% 63%	1600
July	1st to 10th	Night	3 2 1 3 3 4 4 3 7% 9% 61%	2580
	11th to 20th	Night	1 2 1 3 5% 5% 6% 5% 8% 10% 54%	2540
	21st to end	Night	1 3 1 4 3 5% 5% 5% 7% 7% 59%	3002
August	1st to 10th	Night	2 2 1 5% 5% 7% 7% 6% 9% 9% 46%	2847
	11th to 20th	Night	2 3 1 5% 5% 5% 6% 5% 9% 11% 47%	2984
	21st to end	Night	2 3 1 5% 4 5% 6% 5% 6% 8% 53%	3487
September	1st to 10th	Night	4 4 1 4 3 4 4 4 5% 7% 59%	1900
	11th to 20th	Night	2 4 2 3 3 2 4 3 5% 7% 64%	1900
	21st to end	Night	3 4 1 4 2 3 3 3 4 7% 66%	2136
October	1st to 10th	Night	2 3 1 3 2 2 3 2 3 5% 74%	3800
	11th to 20th	Night	2 4 1 5% 2 3 4 3 5% 9% 61%	3778
	21st to end	Night	4 6% 2 5% 3 3 3 3 5% 6% 62%	4120
November	1st to 10th	Night	3 7% 3 6% 4 3 3 3 6% 7% 57%	2514
	11th to 20th	Night	2 9% 3 6% 4 3 3 3 5% 7% 54%	2609
	21st to end	Night	2 7% 3 8% 4 4 4 4 6% 9% 49%	2800
December	1st to 10th	Night	1 6% 4 9% 5% 3 4 7% 16% 7% 38%	4570
	11th to 20th	Night	1 7% 4 7% 5% 6% 7% 6% 10% 10% 38%	5181
	21st to end	Night	1 9% 5% 13% 8% 7% 7% 6% 10% 9% 23%	6974

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	44% 9% 16% 20% 10% 1	3639
	11th to 20th	Day	44% 11% 12% 19% 13% 1	3936
	21st to end	Day	36% 9% 14% 23% 15% 4	4510
February	1st to 10th	Day	46% 10% 15% 16% 11% 2	1800
	11th to 20th	Day	44% 10% 12% 21% 11% 2	1849
	21st to end	Day	43% 8% 16% 15% 15% 3	1650
March	1st to 10th	Day	34% 7% 10% 26% 18% 6%	4656
	11th to 20th	Day	31% 8% 12% 36% 12% 1	5000
	21st to end	Day	40% 13% 16% 23% 6% 1	5678
April	1st to 10th	Day	36% 9% 14% 22% 14% 5%	2400
	11th to 20th	Day	43% 7% 15% 16% 17% 2	2374
	21st to end	Day	40% 10% 11% 26% 12% 1	2400
May	1st to 10th	Day	39% 11% 14% 19% 15% 2	5804
	11th to 20th	Day	31% 10% 13% 25% 18% 4	6302
	21st to end	Day	27% 9% 14% 24% 24% 3	6926
June	1st to 10th	Day	24% 9% 14% 23% 22% 8%	2800
	11th to 20th	Day	21% 7% 10% 18% 26% 18%	2800
	21st to end	Day	20% 6% 9% 22% 26% 17%	2800
July	1st to 10th	Day	19% 8% 7% 15% 26% 25%	4432
	11th to 20th	Day	24% 11% 11% 23% 19% 12%	4355
	21st to end	Day	25% 10% 10% 20% 19% 18%	4413
August	1st to 10th	Day	30% 10% 18% 17% 14% 11%	4078
	11th to 20th	Day	29% 9% 15% 19% 20% 7%	3892
	21st to end	Day	24% 8% 14% 21% 24% 10%	4179
September	1st to 10th	Day	24% 7% 8% 19% 27% 15%	2280
	11th to 20th	Day	23% 8% 12% 22% 26% 10%	2280
	21st to end	Day	22% 5% 7% 30% 29% 7%	2041
October	1st to 10th	Day	19% 5% 5% 26% 32% 13%	3170
	11th to 20th	Day	24% 5% 12% 34% 22% 3	3150
	21st to end	Day	29% 7% 10% 37% 15% 2	3438
November	1st to 10th	Day	28% 5% 15% 27% 24% 1	1763
	11th to 20th	Day	31% 7% 16% 26% 17% 2	1789
	21st to end	Day	32% 5% 14% 33% 14% 3	1600
December	1st to 10th	Day	33% 8% 16% 33% 9% 1	2620
	11th to 20th	Day	29% 8% 22% 28% 13%	3024
	21st to end	Day	42% 7% 16% 21% 11% 2	4004
January	1st to 10th	Night	45% 10% 17% 19% 8% 1	6327
	11th to 20th	Night	48% 11% 12% 16% 11% 1	6087
	21st to end	Night	41% 12% 14% 17% 14% 2	6468
February	1st to 10th	Night	44% 10% 16% 17% 10% 2	2600
	11th to 20th	Night	41% 9% 13% 24% 11% 2	2448
	21st to end	Night	44% 6% 12% 19% 16% 2	1980
March	1st to 10th	Night	36% 9% 8% 26% 18% 2	5208
	11th to 20th	Night	28% 6% 13% 32% 19% 2	5000
	21st to end	Night	35% 11% 18% 30% 5% 1	5274
April	1st to 10th	Night	33% 9% 15% 23% 16% 3	2000
	11th to 20th	Night	37% 7% 14% 22% 18% 2	1977
	21st to end	Night	33% 8% 13% 28% 18%	2000
May	1st to 10th	Night	34% 9% 18% 26% 12% 1	4168
	11th to 20th	Night	28% 10% 15% 30% 15% 2	3618
	21st to end	Night	23% 9% 14% 28% 23% 3	3972
June	1st to 10th	Night	18% 7% 17% 29% 21% 8%	1600
	11th to 20th	Night	18% 7% 12% 18% 24% 21%	1600
	21st to end	Night	15% 6% 9% 24% 26% 19%	1600
July	1st to 10th	Night	18% 7% 11% 20% 18% 27%	2540
	11th to 20th	Night	19% 9% 20% 26% 17% 9%	2492
	21st to end	Night	19% 8% 15% 28% 13% 16%	2908
August	1st to 10th	Night	27% 11% 19% 20% 14% 10%	2838
	11th to 20th	Night	27% 12% 19% 15% 17% 10%	3011
	21st to end	Night	27% 8% 12% 18% 26% 9%	3487
September	1st to 10th	Night	23% 6% 10% 20% 25% 16%	1900
	11th to 20th	Night	19% 5% 14% 23% 25% 14%	1900
	21st to end	Night	20% 5% 8% 28% 31% 8%	2136
October	1st to 10th	Night	15% 4 7% 29% 35% 11%	3800
	11th to 20th	Night	20% 4 16% 37% 18% 3	3778
	21st to end	Night	23% 6% 11% 39% 20% 2	4120
November	1st to 10th	Night	27% 5% 13% 29% 23% 3	2514
	11th to 20th	Night	29% 7% 15% 30% 17% 2	2609
	21st to end	Night	31% 7% 13% 31% 16% 2	2800
December	1st to 10th	Night	36% 17% 14% 25% 8% 1	4570
	11th to 20th	Night	29% 10% 22% 29% 9%	5182
	21st to end	Night	44% 11% 15% 16% 13% 1	6974

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	9% 16% 24% 32% 14% 4	300
	11th to 20th	Day	10% 17% 29% 33% 10% 2	300
	21st to end	Day	4 12% 35% 32% 14% 3	330
February	1st to 10th	Day	13% 25% 35% 21% 5% 1	300
	11th to 20th	Day	12% 24% 32% 28% 4 1	300
	21st to end	Day	16% 23% 29% 25% 8%	249
March	1st to 10th	Day	12% 25% 30% 21% 10% 2	365
	11th to 20th	Day	11% 24% 31% 23% 9% 1	500
	21st to end	Day	13% 34% 31% 16% 6% 1	550
April	1st to 10th	Day	15% 25% 32% 17% 8% 2	500
	11th to 20th	Day	15% 34% 27% 18% 5% 1	492
	21st to end	Day	13% 25% 33% 18% 9% 2	500
May	1st to 10th	Day	15% 22% 31% 20% 9% 2	547
	11th to 20th	Day	12% 13% 28% 29% 15% 2	725
	21st to end	Day	9% 14% 28% 28% 17% 3	789
June	1st to 10th	Day	6% 13% 24% 30% 21% 6%	730
	11th to 20th	Day	6% 13% 29% 33% 18% 2	730
	21st to end	Day	4 12% 24% 34% 23% 3	730
July	1st to 10th	Day	5% 6% 18% 34% 32% 5%	697
	11th to 20th	Day	4 5% 20% 32% 33% 6%	668
	21st to end	Day	6% 4 19% 34% 28% 9%	595
August	1st to 10th	Day	8% 6% 26% 33% 21% 7%	487
	11th to 20th	Day	4 6% 26% 36% 20% 8%	477
	21st to end	Day	4 8% 26% 34% 21% 7%	539
September	1st to 10th	Day	6% 11% 22% 28% 22% 11%	490
	11th to 20th	Day	4 12% 33% 28% 17% 6%	490
	21st to end	Day	7% 11% 24% 25% 23% 9%	338
October	1st to 10th	Day	9% 10% 21% 23% 24% 12%	300
	11th to 20th	Day	7% 19% 20% 29% 18% 6%	300
	21st to end	Day	12% 18% 26% 25% 15% 2	330
November	1st to 10th	Day	11% 18% 26% 28% 14% 2	300
	11th to 20th	Day	11% 16% 28% 29% 13% 3	300
	21st to end	Day	6% 12% 34% 32% 15% 1	300
December	1st to 10th	Day	9% 17% 26% 30% 14% 4	300
	11th to 20th	Day	6% 16% 29% 40% 10%	300
	21st to end	Day	10% 16% 38% 27% 8% 2	330
January	1st to 10th	Night	6% 16% 27% 38% 13% 1	1000
	11th to 20th	Night	7% 13% 34% 35% 10% 1	890
	21st to end	Night	4 11% 34% 41% 10% 1	880
February	1st to 10th	Night	6% 11% 42% 34% 7% 1	800
	11th to 20th	Night	4 12% 41% 35% 7%	784
	21st to end	Night	6% 11% 41% 38% 4	660
March	1st to 10th	Night	5% 10% 36% 39% 10%	800
	11th to 20th	Night	3 9% 30% 45% 13%	800
	21st to end	Night	5% 8% 37% 41% 9%	794
April	1st to 10th	Night	8% 12% 37% 39% 5%	600
	11th to 20th	Night	8% 12% 34% 38% 8% 1	593
	21st to end	Night	8% 12% 28% 40% 11% 1	600
May	1st to 10th	Night	7% 10% 28% 43% 11% 1	600
	11th to 20th	Night	10% 3 21% 46% 19% 1	591
	21st to end	Night	6% 7% 24% 40% 22% 1	650
June	1st to 10th	Night	4 5% 18% 46% 26% 1	600
	11th to 20th	Night	3 5% 25% 51% 16% 1	600
	21st to end	Night	3 3 18% 49% 27%	600
July	1st to 10th	Night	3 2 9% 40% 43% 2	570
	11th to 20th	Night	1 1 5% 47% 43% 2	552
	21st to end	Night	4 1 6% 42% 44% 3	624
August	1st to 10th	Night	5% 2 7% 43% 42% 1	567
	11th to 20th	Night	4 1 9% 52% 32% 2	564
	21st to end	Night	5% 8% 45% 39% 3	627
September	1st to 10th	Night	4 2 11% 51% 30% 2	570
	11th to 20th	Night	4 2 13% 52% 27% 2	570
	21st to end	Night	5% 4 18% 43% 28% 2	655
October	1st to 10th	Night	6% 5% 22% 43% 22% 2	760
	11th to 20th	Night	6% 7% 28% 43% 16% 1	760
	21st to end	Night	10% 8% 24% 40% 15% 2	836
November	1st to 10th	Night	6% 9% 33% 39% 12% 1	954
	11th to 20th	Night	8% 11% 34% 36% 9% 1	1000
	21st to end	Night	5% 10% 32% 41% 12% 1	1000
December	1st to 10th	Night	6% 12% 35% 37% 10% 1	1000
	11th to 20th	Night	6% 12% 36% 36% 9% 1	1000
	21st to end	Night	6% 16% 35% 34% 8% 1	1100

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